In self-biased Phase-Locked Loops (PLLs) the Voltage Controlled Oscillator (VCO) will typically have very high gain, on the order of a few GHz per volt. With wide variations in operating conditions and silicon processing, the control voltage for the VCO can go through a wide range while trying to achieve lock. If the control voltage is left unlimited, the very high VCO gain can cause the VCO output frequency to go too high, to a region where the VCO itself does not have the sufficient open-loop gain to sustain oscillations. The control loop of the PLL then forces the control voltage to make the VCO run even faster, thus causing it to stay out of lock and not recover.
Prior art solutions employ either fixed limits for the VCO control voltage or ones that only track power supply changes. These solutions must keep the VCO control voltage window much wider to account for process variations. This can force the VCO into a region where it will not oscillate.
The top portion of FIG. 1 shows a typical self-biased PLL based on “Low-Jitter Process-Independent DLL and PLL Based on Self-Biased Techniques”, J. G. Maneatis, Journal of Solid State Circuits, November 1996. The top portion of FIG. 1 includes input Fin; phase frequency detector (PFD) 20; dual charge pumps 22; bias generator 24; VCO 26; current mode logic (CML) dividers 28; CML to CMOS converter 30; output Fout; and source voltage VDD. To keep the VCO control voltage VCTRL from entering regions where the VCO may not oscillate, the circuitry in the lower portion of FIG. 1 has been added. The lower portion of FIG. 1 includes control voltage VCTRL comparators 32 and 34; switches 36 and 38; variable VCTRL window generator 40; and safe VCTRL generator 42 for Pull-in. If the control voltage VCTRL is outside the lower and upper control voltage limits Vcmpl and Vcmph, respectively, one of the two switches 36 and 38 is closed. This will pull voltage VCTRL into the operating window towards a target voltage. Region 44 of FIG. 1 provides limits for the control voltage.
Previous designs typically use a resistor divider from the power supply to generate control limits Vcmpl and Vcmph. This will somewhat track power supply changes, but does not follow process variations. This is not adequate for extreme operating conditions, such as automotive applications where the junction temperature can be −40 to 150° C.